One class of nonvolatile memory devices includes phase-changeable random access memory (PRAM) devices, which offer many advantageous electrical characteristics relative to FLASH, SRAM and DRAM memory devices. PRAM devices support non-volatile data storage, random access addressing and relatively high speed read and write operations. PRAM devices may also be configured to have relatively low power consumption requirements.
The nonvolatile characteristics of the PRAM devices may be provided by configuring each memory cell with a chalcogenide alloy (e.g., GST: Ge2Sb2Te5) having programmable resistivity characteristics. For example, during a write/programming operation, the chalcogenide alloy within a memory cell may undergo resistive heating to thereby alter the resistivity of the chalcogenide alloy and cause the memory cell to be “set” into one logic state or “reset” into another logic state.
FIG. 1 illustrates a conventional diode-type PRAM cell 10, which is electrically coupled to respective bit and word lines (BL and WL). In this PRAM cell 10, the chalcogenide alloy (e.g., GST alloy) may be programmed to have a relatively high resistance state (high-R state) or a relatively low resistance state (low-R state). This state may be detected during a reading operation by biasing the bit line BL at a higher voltage relative to the word line to thereby establish a forward current path through the PRAM cell 10. The magnitude of the established current (e.g., bit line current) in the forward current path is measured to determine the state (high-R or low-R) of the cell 10.